The present invention relates to aqueous fluoropolymer dispersions to be used in coating for surfaces, preferably metal and ceramic surfaces, and in the textile-impregnation and cast film manufacture.
Specifically, the present invention relates to aqueous fluoropolymer dispersions able to give films having an high critical thickness and an acceptable shelf life for industrial cycles, not lower than 6 months. The films obtained from these dispersions show good mechanical properties also at high temperatures, good optical properties, as well as reduced porosity and roughness of the film surface.
With critical thickness the maximum thickness obtainable without cracks and surface defects in the film, is meant.
It is very important from the application point of view to increase the limit of the critical thickness so as to have films free from cracks in correspondence of an higher thickness. A higher critical thickness allows greater freedom degrees in formulating and applying formulations with the various technologies and a higher reliability of the product in the transformation process. An higher critical thickness means high productivity in coating industrial cycles for metal surfaces, impregnation and cast films. This feature however must not decrease the mechanical and optical properties.
It is known in the prior art that dispersions of polymers with broadened particle size distribution (PSD) show improved Theological characteristics, in particular a lower viscosity, the solid fraction by volume in the dispersion being equal, from the point of view of their application in various industrial fields. See for example J. of Applied Polymer Sci., 15, 2007-2021 (1971) and Polymer, 33 (22), 4832-4837 (1992). This concept has been applied for obtaining dispersions with bimodal or multimodal PSDs. For example for styrene-butadiene rubber latexes (SBR) see U.S. Pat. No. 4,657,966; for acrylonitrile-butadiene-styrene (ABS) latexes see U.S. Pat. No. 4,385,157; for rubber latex blends see U.S. Pat. No. 4,334,039. As prior art scientific papers, J. of Applied Polymer Sci., 70, 2667-2677 (1998), Colloid Polymer Sci. 276, 305-312 (1998), Colloid Polymer Sci. 275, 986-991 (1997), J. of Rheology 32, 751-771 (1988), can be mentioned.
The same concept applied to the fluoropolymer aqueous dispersions is reported in EP 657,514. This patent describes the use of fluoropolymer dispersion mixtures, expressly excluding thermoplastic polymers, obtained by emulsion polymerization and mixed for obtaining bimodal particle size distributions in order to optimize, specifically in the textile impregnation field, the polymer amount applied in each passage without crack formation. The ratio between the particle sizes having a smaller diameter and the particle sizes having a greater diameter is in the range 0.3-0.7. The first dispersion has a particle number average diameter in the range 180-440 nm, while the second fluoropolymer dispersion has an average diameter in the range 50-150 nm. The amount of the component having the smaller diameter compared with the component with the greater diameter is in the range 5-50% by weight, preferably 5-20% by weight. The examples reported in this patent substantially relate to the textile impregnation and show that, by using an amount of 10% and 18% by weight of the component having a smaller size, cracks are eliminated and the fluoropolymer amount applied in each passage increases. If amounts other than these two values are used, cracks are noticed. In the Examples, the polymer having a smaller size, has particle sizes in the range 100-110 nm and the above mentioned ratio ranges from 0.45 to 0.5. The only example given on the metal coating shows that with an amount equal to 10% of the fluoropolymer having smaller size particles, the film hardness increases compared with the case where the film is obtained by solely using the fluoropolymer having the high sizes as above defined.
The Applicant has tried to reproduce the Examples by using the small particle sizes towards the lowest value mentioned in the patent, also by using for example a larger amount of surfactant with respect to the patent teaching and to the amounts mentioned in the patent Examples. The Applicant has also made other attempts to reproduce the patent by extrapolating the teachings beyond the lowest ratio limit between small and great particles indicated in the patent (lower than 0.3). All these attempts of the Applicant have caused instability phenomena of the latex, both as such and stabilized with non ionic surfactants, as well as a global property worsening, such as for example critical thickness, mechanical properties, film gloss, after a storage of even 1-2 days or 1-2 weeks. This represents a limit from the application point of view since the dispersions should be prepared and used in a short period of time, incompatible with the industrial cycles. In any case said latexes after 2-3 months ageing do not allow to maintain the initial properties as above indicated. From the industrial point of view said latexes have a poor utility.
The same PSD concept has been applied for bimodal distributions in the patent WO 98/58984 wherein, in comparison with EP 657,514, a fluorinated thermoplastic polymer as one of the mixture components is used. The teaching of this patent allow to materialize the favourable bimodality effects, extended towards small particle populations under 80 nm, but it is limited and specific for thermoprocessable polymers, therefore with the drawback to worsen the mechanical properties at high temperatures.
Processes of the prior art to obtain small fluoropolymer particles under 100 nm use high surfactant amounts. See for example EP 248,446 and EP 369,466 relating to polytetrafluoroethylene (PTFE) particles. The obtained particles are anisotropic with a length-diameter L/D ratio higher than 5: these particles are commonly called fibrils.
The need was felt to obtain aqueous fluoropolymer dispersions capable to give films having an high critical thickness combined with good optical and mechanical properties also at high temperatures, reduced porosity and roughness and having an industrially acceptable shelf life, i.e., such as to maintain said properties after a prolonged dispersion storage of at least 6 months.
The Applicant has unexpectedly and surprisingly found that the instability (shelf-life) problems and the non obtainment of the desirable film performances (see above) are correlated to the presence of fibrils in the small size particle population. The Applicant has found that it is possible to solve the above mentioned technical problem by using the dispersions as defined hereinunder.
It is therefore an object of the present invention fluoropolymer dispersion mixtures comprising:
a) one or more dispersions formed by tetrafluoroethylene (TFE) homopolymers or by its copolymers with one or more monomers containing at least one unsaturation of ethylene type in amounts from 0 to 8% by weight, preferably from 0.01 to 3% by weight; the average particle sizes range from 180 to 400 nm, preferably from 200 to 300 nm;
b) one or more tetrafluoroethylene (TFE) dispersions with one or more monomers containing at least one ethylene unsaturation, the comonomer amount being such that the dispersion contains a fibril number lower than 10% of the particle total number, preferably lower than 5%, still more preferably lower than 1%, said fibrils being polymer particles having a length/diameter (L/D) ratio higher than 5; the fibrile number determination is carried out with an atomic force microscope (AFM); the dispersion particle average sizes are smaller than about 90 nm, preferably in the range 10-80 nm, more preferably 20-60 nm; component b) contains a comonomer amount such that the resulting polymer is not elastomeric and furthermore it shows composition and viscosity of the melt such as to be non thermoprocessable.
The dispersion mixture formed by a) and b) generally shows composition and viscosity of the melt such as to be non thermoprocessable.
As said the dispersion b) contains a fibril number lower than 10% of the particle total number, preferably lower than 5%, still more preferably lower than 1%. It has been found by the Applicant that the dispersions containing said fibrils in higher amounts, even though said dispersions are initially effective in increasing the critical thickness, show during the time, from few days to 1-2 months depending on the cases, a decrease of the critical thickness increase, as well as of the other performances until elimination of the advantages deriving from the use of a bimodal (polydispersed) mixture. Besides, instability problems of the dispersion and formulations obtained therefrom during the coating application arise, problems which makes the dispersions unusable.
According to a theory, however non binding, the Applicant thinks that the fibrils present in the dispersion tend to form separated phase and to link each other and with the spheroidal particles of the larger particle dispersion, giving rise to instability sites which obstacle a correct application of the dispersion to give good quality coatings having the combination of the above mentioned properties. These separation and aggregation phenomena take place both when the dispersion is that directly obtained from the polymerization, and when it is further stabilized with surfactants.
The Applicant has found that in order to obtain the present invention results, it is necessary to have at least two dispersions having the above particle sizes distributions and having a fibril percentage in the dispersion b) lower than that above mentioned.
The weight ratio between component a) and component b) as dry product ranges between 99/1 and 80/20, preferably between 99/1 and 90/10, more preferably between 92/8 and 95/5.
The dispersion are usually used at a concentration of the dry product in the range 25%-75% by weight and preferably 40%-65% by weight.
The mixture can be obtained by simple mixing the component a) previously concentrated by the known methods (addition of non ionic surfactant and heating or ultrafiltration) with the component b) so as it is obtained from the polymerization autoclave or concentrated as above for component a) or it can be obtained by coconcentration of the two latexes.
The ratio between the dispersion b) particle sizes compared with those of the dispersion a) is preferably lower than 0.3, more preferably in the range 0.1-0.25.
Furthermore the Applicant has found that the widening of the dispersion b) particle size distribution until comprising the population tails of the dispersion a), leads to a more constant increase of critical thickness in a wider range of compositions. Said widening can be obtained by mixing more dispersions of type b) having a different average diameter.
Among the TFE comonomers one can cite the fluorinated ones. Examples of the latter are:
perfluoroolefins C3-C8, such as hexafluoropropene (HFP);
hydrogenated fluorooelfins C2-C8, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a perfluoroalkyl C1-C6; chloro- and/or bromo- and/or iodo-fluoroolefins C2-C8, such as chlorotrifluoroethylene (CTFE);
(per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a (per)fluoroalkyl C1-C6, for example CF3, C2F5, C3F7;
(per)fluoro-oxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is: an alkyl C1-C12, or an oxyalkyl C1-C12, or a (per)fluoro-oxyalkyl C1-C12 having one or more ether groups, for example perfluoro-2-propoxy-propyl;
fluorodioxoles, preferably perfluorodioxoles;
non conjugated diens of the type:
CF2xe2x95x90CFOCF2CF2CFxe2x95x90CF2,
CFX1xe2x95x90CX2OCX3X4OCX2xe2x95x90CX1F
wherein X1 and X2, equal to or diferrent from each other, are F, Cl or H; X3 and X4, equal to or different from each other, are F or CF3, which during the polymerization cyclopolymerize.
Generally, the comonomer amount in the polymer of dispersion b) is in the range of about 0.5-20% by weight, preferably 1.5-18% by weight, and it is in connection with the kind of comonomer. The skilled in the field is easily capable by routine tests to determine the comonomer amount to have a non thermoprocessable polymer, that is, not workable from the melt (non thermomouldable by extrusion in manufactured articles).
The Applicant has found that the best results are obtained with the contemporaneous use of its polymerization technology in fluorinated microemulsion, described in the European patent application EP 99112083.3 in the name of the Applicant, and of comonomers having an high modifying capability; for example (per)fluorodioxoles having the formula (I): 
are preferred, wherein Yxe2x80x2=xe2x80x94H, xe2x80x94Cl, xe2x80x94F, xe2x80x94CF3, xe2x80x94Oxe2x80x94CF3; X1 and X2 equal to and different from each other are F or CF3. Preferably Yxe2x80x2=xe2x80x94Oxe2x80x94CF3 and X1, X2 =F since with the synthesis process described in EP 633,257 for these comonomers a structured 80/20 sin/anti dioxole is obtained.
The preferred comonomers according to the present invention are those which do not substantially lower the PTFE thermal stability and the molecular weight.
Alternatively to the dioxoles of formula (I), dienes as above mentioned which cyclopolymerize during the polymerization, can be used.
The dioxole class (I) allows to obtain dispersions with average dimension, measured by PCS, from 20 nm to 80 nm, practically free from fibrils and with polymer dry fractions higher than 20% by weight, preferably higher than 25% by weight. The polymer forming the dispersions b) contains an amount of comonomer (I) preferably in the range 1.1%-3% by weight, more preferably 1.5-2.5% by weight.
A further advantage of the perfluorinated comonomer class (I) is the capability not to give monomolecular termination reactions when the comonomer has entered the macromolecular chain and therefore to allow the obtainment of high molecular weights, such as to guarantee improved mechanical properties, especially at high temperatures, even higher than 200xc2x0 C.
The Applicant has found that it is possible to limit the fibril formation under the mentioned values, also by polymerizing under such conditions so as to obtain the dispersion b) with average size particles over 100 nm. However these dispersions do not produce the positive effects combined with the bimodal (polydispersed) distributions.
By using as comonomers perfluoroalkylvinylethers, in the case of perfluoropropylvinylether (PVE) the amount ranges from 3 to 5% by weight; in the case of mixture PVE with perfluoromethylvinylether (MVE) an amount from 4 to 10% by weight, preferably from 3 to 5% by weight, can be used.
The molecular weight of the polymer of dispersion b) can be regulated by conventional transfer agents, for example ethane. The molecular weight can also be regulated through the polymerization initiator amount. However it is to be noticed that by lowering the molecular weight the mechanical properties at high temperatures are worsened.
It is to be noticed that using comonomers different from the dioxoles (I), high molecular weights necessary to have the very good mechanical properties of the present invention, are not obtained.
As known, the viscosity of the melt, so that the polymer is not thermoprocessable, is generally higher than 109 Pas.
Also copolymers having a viscosity in the range 103-109 Pas can be used, provided that the composition is such to make the copolymer non thermoprocessable.
The combination of the properties obtainable by using the present invention dispersions, in particular a remarkable increase of the critical thickness, could be explained, without to be bound to any theory, by hypothetizing a mechanism whereby the particles of dispersion b) do not statistically link together with the larger particles (dispersion a)), but they preferably segregate between each other and take up interstitial spaces and favour at any rate a greater aggregation density in the film formation critical step, i.e. during the drying phase.
This would lead to a thicker structure which would explain an higher resistance to the crack formation which generally takes place during the film drying phase, when there are strong biaxial stresses inside the film.
The type a) aqueous dispersions are obtainable with the conventional emulsion polymerization processes.
The type b) dispersions are obtainable with the process described in detail in the European patent application EP 99112083.3, herein incorporated by reference. Specifically it comprises the following steps:
a) preparation of a perfluoropolyether (PFPE) aqueous microemulsion having perfluorinated end groups or end groups optionally containing 1 or more H atoms, Cl instead of fluorine;
b) feeding of the microemulsion to the polymerization reactor in such amount wherefore the microemulsion perfluoropolyether oil phase is present in a concentration higher than 2 ml for liter of reaction medium;
c) feeding of the reaction medium to the polymerization reactor, reactor degassing, reactor pressurization with gaseous TFE, optional addition of surfactants, stabilizers, comonomes, transfer agents;
d) addition of the initiator, and optionally during the polymerization, of further amounts of surfactants, stabilizers, comonomers, transfer agents;
e) discharge from the reactor of the polymeric latex.
The microemulsion feeding mentioned at point b) can also be carried out after feeding of the reation medium and of the other ingredients mentioned at point c).
Besides the components mentioned at points c) and d) other components commonly used in the TFE polymerization can also be added. For example, polymerization inhibitors, buffers, etc., can be mentioned.
During the polymerization additional initiator and the other components mentioned in c) and in d) can be added even though they have already been introduced into the reactor at the beginning of the reaction.
As said, it has been unexpectedly found by the Applicant that it is essential to refer not only to the average diameter of the particle size distribution, but also to the shape factor of the obtained polymer primary particles. Specifically, it is important that most particles have a spheroidal shape and that the number of fibrils is lower than the above mentioned limits.
The microemulsions used in the process of the present invention are described in U.S. Pat. No. 4,864,006 and U.S. Pat. No. 4,990,283, herein incorporated by reference, wherein instead of the mentioned perfluoropolyethers having non reactive end groups, also hydrofluoropolyethers having one or both end groups containing one hydrogen atom or having one or more chlorine atoms instead of fluorine in the chain end groups, can be used. The surfactants which can be used both for preparing the microemulsion and during the polymerization, are those described in the mentioned patents or those having an end group wherein one or more fluorine atoms are substituted by chlorine and/or hydrogen. The PFPE molecular weight which can be used can also be lower than 500, for example even 300, as average molecular weight by number. The nanoemulsions obtained by the use of PFPE having a low molecular weight, in the range 350-600, preferably 350-500, can more advantageously be used in the applications wherein their quantitative removal is required.
The total surfactant amount used is such that the weight ratio between surfactant and TFE converted into polymer is preferably lower than 1.
The copolymer molecular weight of the dispersion b) obtained by the present invention process is such as to give a good chemical and thermal stability of the polymer. Generally the obtained molecular weights are higher than 50,000, for example 500,000-5,000,000.
During the polymerization, the temperatures and pressures conventional of the TFE polymerization processes are used.
As already said, with the invention dispersions the gloss and the scratch resistance of the obtained films, also at temperatures higher than 200xc2x0 C., have improved. Particularly high gloss values are also obtainable by increasing the comonomer amounts in the small particle (b) populations, as well as in the great particle (a) populations. Preferably, as comonomers, the dioxoles of formula (I) which allow to obtain high molecular weights (in the range 500,000-5,000,000) are used.
The obtained dispersion mixture can be suitably formulated in connection with the specific application with the addition of other aqueous resin dispersions, such as, acrylic resins, silicone resins, polyamidoamide resins, imide resins, etc.; pigments, surfactants, inorganic fillers and other additives, such as antifoam agents, extending agents, etc. After the mixture application to the desired surface, the film is dried and then sintered at a temperature higher than the polymer melting temperature.
The total surfactant amount necessary to stabilize the invention dispersion mixture generally ranges from 2 to 10% by weight, and it is preferably in the range 3-6% on the dispersion weight.
The fluoropolymer aqueous dispersions of the present invention, besides for coating applications on metal surfaces, can also be used for ceramic surfaces and in the textile impergnation and for obtaining cast films.
The invention dispersions as said allow a remarkable improvement in the film formation without the presence of cracks (greater critical thickness). However this higher property is not fully exploitable in the case of coatings of a vertical support, as it is the typical case of the textile impregnation, the cast film manufacture. Indeed the total amount of deposited solid depends on the wetting capability and on the rheological properties of the dispersion.
The Applicant has surprisingly found that the advantage of the better filming (absence of cracks) can be fully exploited by adding to the invention dispersions a non ionic fluorinated surfactant having the formula: 
n is an integer in the range 4-60, preferably 8-30;
L and Lxe2x80x2, equal to or different from each other, are selected from: 
 wherein: p is 0 or 1; Y can be F or CF3;
Rxe2x80x2 is a C1-C5 alkylene radical;
R1, R2 can be both H or the one H and the other CH3.
Rf and Rfxe2x80x2, equal to or different from each other, can be perfluoropolyether radicals having a number average molecular weight in the range 250-1,500, preferably 400-1,000; perfluorocarbon radicals having the above mentioned average moelcular weight.
The Rf and Rfxe2x80x2 perfluoropolyether radicals comprise a T end group and repeating units statistically distributed along the polymer chain selected from: 
or xe2x80x94CFXOxe2x80x94, wherein X is equal to F or xe2x80x94CF3;
xe2x80x94CF2(CF2)zOxe2x80x94 wherein z is an integer equal to 2 or 3;
xe2x80x94CF2CF(ORfxe2x80x3)Oxe2x80x94 or xe2x80x94CF(ORfxe2x80x3)Oxe2x80x94 wherein Rfxe2x80x3 can be xe2x80x94CF3,
xe2x80x94C2F5, or xe2x80x94C3F7.
The perfluoropolyether radical T terminal is selected from: xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7, ClCF2CF(CF3)xe2x80x94, CF3CFClCF2xe2x80x94, ClCF2CF2xe2x80x94 and ClCF2xe2x80x94, CF3CFHCF2xe2x80x94, HCF2CF2xe2x80x94 and HCF2xe2x80x94.
In particular the following Rf and Rfxe2x80x2 perfluoropolyether radicals can be mentioned:
(a) Txe2x80x94O(CF2CF(CF3)O)a(CFXO)bxe2x80x94
wherein: X is F or CF3; a and b are integers such that the molecular weight is in the above mentioned range; a/b is in the range 10-100, and T is one of the above mentioned end groups;
(b) Txe2x80x94O(CF2CF2O)c(CF2O)d(CF2(CF2)zCF2O)hxe2x80x94
wherein: c, d and h are integers such that the molecular weight is in the above mentioned range; c/d is in the range 0.1-10; h/(c+d) is in the range 0-0.05, z is an integer equal to 2 or 3, and T is one of the above mentioned end groups;
(c) Txe2x80x94O(CF2CF(CF3)O)e(CF2CF2O)f(CFXO)gxe2x80x94
wherein: X is F or CF3; e, f, g are integers such that the molecular weight is in the above mentioned range; e/(f+g) is in the range 0.1-10, f/g is in the range 2-10, T is one of the above mentioned end groups;
(d) Txe2x80x94O(CF2O)j(CF2CF(ORfxe2x80x3)O)k(CF(ORfxe2x80x3)O)lxe2x80x94
wherein: Rfxe2x80x3 is xe2x80x94CF3, xe2x80x94C2F5, xe2x80x94C3F7; j, k, l are integers such that the molecular weight is in the above mentioned range; k+1 and j+k+1 are at least equal to 2, k/(j+1) is in the range 0.01-1,000, l/j is in the range 0.01-100; T is one of the above mentioned end groups;
(e) Txe2x80x94Oxe2x80x94(CF2(CF2)zCF2O)sxe2x80x94
wherein: s is an integer such as to give the above mentioned molecular weight, z has the above defined meaning and T is one of the above mentioned end groups;
(f) Txe2x80x94O(CR4R5CF2CF2O)jxe2x80x2xe2x80x94wherein: R4 and R5 are equal to or different from each other and selected from H, Cl or perfluoroalkyl, for example having 1-4 C atoms, jxe2x80x2 being an integer such that the molecular weight is that mentioned above;
(g) Txe2x80x94O(CF(CF3)CF2O)jxe2x80x3xe2x80x94
jxe2x80x3 being an integer such as to give the above mentioned molecular weight.
These compounds and the methods for their preparation them are described in the patents GB 1,104,482, U.S. Pat. No. 3,242,218, U.S. Pat. No. 3,665,041, U.S. Pat. No. 3,715,378 and U.S. Pat. No. 3,665,041, EP 148,482 and U.S. Pat. No. 4,523,039, U.S. Pat. No. 5,144,092.
The preferred perfluoropolyether radicals of the present invention have the following structures:
Rfxe2x80x3xe2x80x94Oxe2x80x94(CF(CF3)CF2O)a(CF2O)b and
ClC3F6O(CF(CF3)CF2O)a(CF2O)b 
wherein the a/b ratio ranges from about 20 to about 40, and Rfxe2x80x3 has the above defined meaning.
The surfactant of formula (Ia) allows to improve rheology and wettability of the support.
Should a further improvement of the wettability be necessary in order to use completely the latex characteristics, the formulation can be additivated with a fluorinated non ionic surfactant having formula (Ib). 
wherein: M=H, CH3; Rf, L, R1, R2, n have the above mentioned meaning for the surfactant (Ia).
The surfactant amount of the formula (Ia) must be such as to lead to the suitable viscosity for the application. Geneally the surfactant amount depends on the component b) and on the optional components present in the dispersion. For example by using as component b) of about 50 nm, present in an amount of 5% by weight (95% by weight of the component a), the effective surfactant amount is in the range of 0.1% by weight.
The non ionic compounds having formulas (Ia) and (Ib) can be added to the dispersion in amounts generally in the range 0.1-5% by weight. When both the surfactants are additivated, the total amount must be lower than 5% by weight.